Conventionally, when manufacturing a diaphragm spring for use in a clutch cover assembly; plural holes 32 and slits 33 have been punched out from a disc-like metal blank 31 by a first press work as illustrated by FIG. 6, and then a circular center hole 34 has been punched out as shown in FIG. 7 by a second press work. Thereafter the blank has been bent to a specified shape as illustrated by FIG. 8. This diaphragm spring is elastically deformed by being pushed and pulled by a release bearing 36 at inner peripheral parts of fingers 35 formed of the slits 33 to cause a clutch disc to be pressed on and separated from a flywheel through, for instance, a pressure plate.
In the conventional manufacturing method, it has been impossible to make a width of the slit 33 smaller than the plate thickness of the blank 31. Furthermore, contact area between an inner peripheral part of the finger 35 and a release bearing 36 is small because of the reduced inner peripheral width of the finger 35 causing a high bearing pressure. With the high bearing pressure the inner peripheral part of finger 35 has worn out quickly.